Isolation valves are well known and are traditionally used in plumbing and heating applications to control the flow of water or other fluid to and from an in-line appliance or piece of equipment for purposes of maintenance, replacement or repair. However, current designs are large and bulky and do not allow for a full fluid flow. This is undesirable for many reasons. In-line appliances are typically disposed in limited spaces, thus the large bulky size of current designs make them cumbersome and difficult to install in limited spaces. In some cases the limited space makes them impossible to install, remove and/or repair. Furthermore, some in-line appliances, e.g. tankless water heaters, are tested by fluid flow rates and temperature calculations. Disadvantageously, the partial fluid flow capability of current designs makes it difficult for the in-line appliance to be accurately tested or subject to maintenance.